1. Field of the Invention
The present invention relates to methods of conditioning substrates, particularly enamel and dentine, to obtain strong and durable adhesive bonding of polymeric materials.
2. Brief Description of Related Art
When a tooth looses its structural integrity, a restoration is necessary to maintain its biological functions.
The polymeric materials designed to replace the lost tooth tissue has to adhere perfectly to the tooth structure in order to avoid gaps and consequent leakage at the tooth--restoration interface.
The accomplishment of this requirement is difficult because of the following factors:
1. The tooth has a heterogeneous physical and chemical structure. The enamel as a hard protective tissue for the softer and vital dentinal tissue, is highly mineralized and inert, while dentine is a complex structure both inorganic and organic in nature, whose vitality is maintained by a continuous flow of the dentinal fluid. These structural differences entail differences in the capacity of adherence.
2. The adherent surface usually is not large enough to ensure a true adhesion by secondary forces.
3. The surface energy and consequently the surface wetability is too low for an intimate contact with the adhesive.
4. The smear layer left on the surface by mechanical preparation of the cavity, as well as the dentinal fluid, may interfere in the adhesion process.
5. The difference in the chemical nature between predominantly inorganic adherent surface and organic adhesive is not in favour of contact by chemical affinity.
6. The contraction stress induced by polymerization shrinkage may prevail over the adhesion strength.
To overcome the above impediments, the tooth surface has to be conditioned prior to any adhesive application.
The treatment of the tooth surface with conditioners is the crucial first step in the adhesion process, and consequently for restoration.
Heretofore the subject of conditioning was the mineral component of the tooth. The conditioner was a mineral acid (HNO.sub.3 or H.sub.3 PO.sub.4) or organic acid (citric acid, maleic acid or EDTA). These acids condition the tooth surface by an "etching" mechanism.
The effects of etching are: removal of the smear layer, removal of a superficial layer of the underlying sound structure, increasing surface area by pitting the enamel and opening the dentinal tubules, and increasing surface energy with a consequent increase of wetability.
The prior art etchant of choice for enamel is phosphoric acid.
This treatment was inconceivable for conditioning dentine, which is a vital, sensitive and complex structure.
The impractical protocol for the differential conditioning of enamel and dentine, as well as the lack of a milder acid as an alternative for H.sub.3 PO.sub.4, led to the so-called "all-etch" technique using dilute (10-37%) H.sub.3 PO.sub.4 solutions or gel on both enamel and dentine. This technique is used now in many bonding systems, in spite of its controversial benefits in bonding, in balance with its detrimental effects.
The detrimental effects of phosphoric acid even in concentrations of 10%, are well documented: loss of enamel of about 5-10 .mu.m thickness, depth of enamel etch up to 50 .mu.m, increase in dentine permeability and subsequent possible bacteria invasion. An increase in dentine wetness due to the toward flow of the dentinal fluid caused by the hypertonic 10-37% H.sub.3 PO.sub.4 solutions, increased potential for pulpal irritation due to the mentioned hypertonicity, potential for denaturation of collagen and possible difference in depth of demineralization and depth of adhesive penetration and consequently the enamel or dentine weakness under the restoration are all appreciated in the art.
In an effort to find a milder acid equally efficient in conditioning both enamel and dentine, a great number of organic acids have been tried, such as: saturated organic acids e.g. formic, acetic, oxalic, succinic, unsaturated organic acids e.g. maleic, methacrylic, itaconic, citraconic, ascorbic; polyfunctional organic acids e.g. citric, lactic, malic, tartaric, pyruvic, glycine, alanine, ethylenediaminotetraacetic [EDTA].
Citric acid and EDTA are the only polyfunctional organic acids which found a practical use, due to their capacity of etching similar with that of H.sub.3 PO.sub.4. (Gluma bonding system and Amalgam bond).
Since the micromechanical retention is well documented and an accepted mechanism of bonding, the target of conditioning was thought to be "well etching, well clean surface" to provide more and longer polymeric tags.
The newer concept of "hybrid layer" and its contribution in bonding, brought attention to the importance of the penetration of the adhesive into the micro spaces created by etching the tooth surface. The wetability of the tooth surface, which was considered a key factor in the adhesion process, proved to be a necessary condition, but insufficient if it is not associated with good penetration. Based on the latest experimental studies, wetability began to differentiate from penetration. The first as a physical process related to the difference in surface energy between adherent and adhesive, and the second as a diffusion process implying not only the physical structure of the adherent but also the chemical affinity between adherent and adhesive.
It was noticed that etching followed by a treatment ("priming") with a hydrophylic organic compound e.g. N-tolylglycine (NTG) and 2-hydroxy ethylmethacrylate (HEMA), may improve the adhesive penetration into the dentinal tubules and intertubular dentine and consequently the bonding strength. This treatment may be considered the second step of conditioning which addresses the organic phase of the tissue, while the first step, etching, is addressed to inorganic phase.
Complementing each other, the two conditioners facilitate the penetration of adhesive, perhaps not only by diffusion through permeabilized tissue but also by chemical affinity.
An attempt for a more conservative method of conditioning by preserving and modifying the smear layer is the use of a mixture of 2.5% nitric acid with either ferric or aluminum oxalate or phosphate-group containing methacrylates ("self-etching primers"). However the lower bond strength values than was expected, suggest, particularly for "self-etching priming", a poor adhesive penetration.
The present invention provides a conservative method of conditioning, able to improve the adhesive penetration and to yield equally strong and reliable bond strength for both enamel and dentine.
The new method of conditioning is conservative, preserving the smear layer. It allows for a minimum removal of the tooth structure during mechanical preparation of a cavity and improves the treatment of cervical erosions, root caries, decalcified tissue, accidental dental splits or fractures.
The new conditioners employed in the method of the invention are biocompatible, mild polyfunctional organic acids, usually found as constituents in plants, fruits and as metabolic intermediaries in the human body. The new conditioners may prevent pulp pathology, secondary caries and tooth sensitivity by their bacteriostatic effects.
The new conditioners are non-toxic and harmless for soft tissue. No special clinical precautious are necessary with their use.
The new method of conditioning is not an alternative for the known method of conditioning with HNO.sub.3, H.sub.3 PO.sub.4, EDTA, citric acid or maleic acid, but an improvement for adhesion of polymeric materials to the tooth structure in the most conservative and bacteriostatic manner.